The PEP cycle involves the simultaneous conversion of oxaloacetate derived from pyruvate to phosphoenolpyruvate (PEP) and the conversion of PEP to pyruvate. This cycle is assessed using a novel method involving a combination of stable and radioisotopes and linear and compartmental modelling. The hypothesis will be examined that the PEP cycle plays an important role in amplifying control of the net production of glucose via gluconeogenesis.